The operating safety of cables formed by wrapped or woven wire strands, commonly referred to as wire rope, often requires that wire ropes be inspected periodically for defects. Until now rope inspections for the loss of metallic cross section (LMA) and local flaws (LF) were thought to be the proper criteria. Loss of metallic cross section measured quantitatively detects loss of wire rope cross section caused by external and internal corrosion and wear. Local flaw measurements qualitatively detect a wide variety of external and internal discontinuities such as broken wires and large scale corrosion pitting. One magnetic device for nondestructive testing of wire rope for detection of both loss of metallic cross section and local flaws is disclosed in my U.S. Pat. No. 4,659,991.
However, modern wire ropes have a tendency to deteriorate internally in more modes than previously contemplated, and the detection of all modes of external and internal deterioration has required a new approach to the nondestructive inspection of wire rope for defects.
Corrosion pitting causes stress concentrations and is extremely insidious since it causes little loss of material with rather small effects on the surface while damaging the deep structure of the metal. The pits on the wire surfaces are often covered by products of the corrosion. Corrosion pitting inhibits free movement of the wires and strands which produces additional stresses in the wires. The increased wire stresses combined with the above mentioned stress concentrations can drastically accelerate the development of fatigue breaks. Corrosion-assisted wear causes wires to corrode uniformly over their entire surface which may reduce their cross-sectional area and cause loose strands. The severity of corrosion often varies along the length of a wire rope. Frequently, corrosion is localized, but nevertheless is dangerous. The extent of corrosion is often difficult to gauge, and, as shown by experience, is usually underestimated.
Internal mechanical defects include broken wires (single and in clusters), and inter-strand nicking. Many ropes suffering from such defects are torque-balanced, multi-strand ropes comprising two or more layers of twisted strands. FIG. 1 shows a cutaway section of such wire rope. Torque-balance is achieved in multi-strand ropes by layering and wrapping inner and outer strands in opposite directions about a core. This type of rope construction limits axial rotation of a freely suspended rope under load. In addition, multi-strand ropes offer flexibility and a wear-resistant surface profile. In single fall crane operations the use of torque-balanced ropes is mandatory.
However, the strands in different layers of these ropes cross over one another at an angle and touch one another. Therefore, when multi-strand ropes bend over sheaves or a drum, they are subject to the combined effect of radial loading, relative motion between wire strands and bending stresses.
Therefore, multi-strand ropes are prone to develop inter-strand nicking as illustrated in FIG. 2 and internal wire breaks as illustrated in FIG. 3. The breaks occur primarily at the interface of the outer and immediately adjacent inner layer of strands with no externally visible signs. The wires in the inner layer typically show nicking and breaks caused by a combination of fluctuating axial wire stresses, inter-strand motions, and fluctuating radial loads. The broken wires B usually show squared-off and z-shaped ends that are typical of fatigue breaks.
In addition, many multi-strand ropes are subject to corrosive environmental conditions. For example, offshore ropes are either immersed in the sea or continually wetted by salt water spray. Heavy use in a marine environment can displace and degrade the rope lubricant. The combined effects of fatigue, corrosion, and lubricant degradation can cause rapid internal deterioration with no externally visible indications where there is no effective form of protection. Since deterioration of torque-balanced rope is not easily detected, failure of the rope is often unexpected.
Similar nicking and fatigue patterns occur in IWRC (Independent Wire Rope Core) ropes. FIG. 4 shows a typical cross-section diagram of such a rope. For IWRC ropes, the wires of the outer strands of the outer wire bundles have a larger diameter than the outer wires of the core. To minimize inter-strand nicking between the outer strands and the IWRC core, the ropes are designed such that the wires of the outer strands and of the core are approximately parallel. This parallel arrangement is usually achieved by a Lang lay construction for the core and an ordinary lay construction for the outer strands.
The result of these geometrical features is that under fluctuating tensile loads the outer IWRC wires are continuously forced into the valleys between the outer wires, and are then released. The mechanism results in secondary bending stresses which lead to large numbers of cores with broken wire strands due to fatigue breaks. The breaks can be very close together and thus form groups of breaks. Eventually, the IWRC can break, or it can completely disintegrate into short pieces of wire about a half lay length long. This condition is commonly called complete rope core failure.
As the IWRC core fails, the outer strands lose the radial support. The lack of support allows the outer strands to bear against each other tangentially. The resulting inter-strand nicking restricts the movement of the strands within the rope. Without the freedom of movement, secondary fatigue breaks in the wires of the outer strands in the strands of the outer bundles will develop at the strand points of tangency. Because the fatigue breaks develop in the valleys between the outer strands, they are called valley breaks.
As another example, spiral strand is made up of concentric layers of wires spirally wound in opposite directions to allow a measure of torque balance. The individual wires in different layers touch locally and at an angle, and the helical geometry within the layers creates radial inter-layer contact forces. When used in mooring applications, spiral strands are subject to fluctuating loads, and especially bending. Depending on the level of axial tension and radius of curvature, spiral strands are subject to interlayer slippage, which causes axial motion between wire strands in different layers combined with tension and torque stresses. Therefore, it is expected that as a result of the geometrical features, wire strands of layers will develop inter-strand nicking and fretting, and eventually, secondary fatigue breaks.
In view of the numerous ways in which wire ropes can fail both externally and internally, it is desirable to be able to inspect or non-destructively test the ropes in both areas. Loss of metallic cross section is certainly one form of inspection that reveals a general weakening of the wire rope due to external wear. External wear can be detected visually, but this type of inspection is limited generally to the outer most portion of the wire rope. Deeper internal inspection is therefore most important. Unfortunately internal inspection techniques for the defects described above are somewhat lacking and need to be more quantitative to provide a more reliable indication of rope safety. It is accordingly an object of the present invention to provide a non-destructive device and method of inspecting wire rope for assessing rope condition externally and internally at a quantitative level.